The present invention relates to super-clean steel; i.e., steel having enhanced cleanliness. More particularly, the present invention relates to super-clean steel including steel for producing bridge cable, hose wire, bead wire, steel cord used as a reinforcing member in a radial tire of an automobile, or a valve spring used in a valve of an engine. Such super-clean steel exhibits excellent cold workability, as well as excellent fatigue properties required of products such as bridge cable, hose wire, bead wire, steel cord, and a valve spring.
Some non-metallic inclusions (hereinafter may be referred to simply as xe2x80x9cinclusionsxe2x80x9d) in steel products exhibit useful effects; for example, MnS in free-cutting steel enhances machinability. However, the majority of inclusions are detrimental; i.e., they may reduce fatigue life, inhibit workability, or serve as starting points of destruction. Examples of such inclusions include inclusions in super-clean steel such as steel used for producing steel cord which is used as a reinforcing member in a radial tire of an automobile, or for producing a valve spring used in a valve of an engine. Particularly, high-melting-point inclusions such as alumina, spinel, and complexes thereof may considerably lower fatigue properties and cold workability, including drawability, workability in stranding, and forgeability. Therefore, steel products have been produced by means of a process in which inclusions are reduced considerably.
Known methods for evaluating cleanliness of steel products; i.e., for evaluating (examining) inclusions in steel products, include the JIS method, the ASTM method, and the MICHELIN method developed by COMPAGNIE GENERALE DES ETABLISSEMENTS MICHELIN. However, evaluation by way of these conventional methods is time consuming, since the methods are carried out by visual examination through an optical microscope. In addition, these methods involve problems in terms of accuracy and error in measurement.
Recently, techniques for producing super-clean steel have been developed, and thus the number of inclusions in super-clean steel has been reduced drastically. Therefore, in the case of the above-described typical evaluation methods making use of an optical microscope, steel products generally do not differ in cleanliness evaluation index, since inclusions rarely appear in a test surface; however, steel products differ considerably in terms of properties, such as cold workability, including drawability and fatigue properties. Thus, correlation between cleanliness evaluation index obtained by use of an optical microscope and properties of a product tends to be lost. When the area of a test surface is increased, correlation between cleanliness evaluation index obtained by use of an optical microscope and properties of a product can be confirmed. However, in this case, a very large test surface area is required, thus making the evaluation time-consuming and expensive.
Incidentally, xe2x80x9cTETSU-TO-HAGANExe2x80x9d 75th year (1989) Vol. 10, p 1897-1904, describes that a cleanliness evaluation method utilizing an electron-beam melting can provide the information equivalent to that obtained from more than 105 visual fields of measurement under the above-described method using an optical microscope. (Hereinafter, an electron beam may be referred to as an xe2x80x9cEB,xe2x80x9d and a cleanliness evaluation method utilizing the electron-beam melting may be referred to as the xe2x80x9cEB method.xe2x80x9d) The EB method is a method wherein an EB is radiated onto a sample (approximately 1-3 g) for melting the sample within a short period of time to thereby cause inclusions to rise to the surface of the sample, and after solidification of the sample, the inclusions in the surface are measured for evaluation (examination) of cleanliness. According to an exemplary embodiment of the method, a sample is placed on a Cu hearth cooled with water and then melted by irradiation with an EB under vacuum atmosphere.
The EB method does not require a very large area of test surface. Thus, the EB method can provide a more accurate evaluation of properties of a sample than the above-described test method using an optical microscope. In addition, the examination can be performed within a short period of time.
With regard to a method for evaluating cleanliness by means of the EB method, Japanese Patent Application Laid-Open (kokai) No. 40082/1993 among others discloses xe2x80x9ca method for melting a sample for inclusion analysisxe2x80x9d wherein inclusions in a sample can be effectively induced to rise to the surface of the sample. In addition, Japanese Patent Application Laid-Open (kokai) No. 151749/1995 discloses xe2x80x9ca method for evaluating inclusions in a wire rodxe2x80x9d wherein a steel product having a particular carbon concentration is subjected to EB melting.
Among methods for evaluating cleanliness by the EB method, the technique disclosed in Japanese Patent Application Laid-Open (kokai) No. 40082/1993 determines only an energy irradiation rate during EB melting. Therefore, when a sample prepared by a melting method disclosed in the above publication is used, the obtained evaluation may reflect not only the effect of high-melting-point inclusions such as alumina, spinel, and complexes thereof, which adversely affect properties of steel products, but also the effect of low-melting-point inclusions such as MnS and SiO2, which rarely affect properties of steel products. Accordingly, this technique cannot be used to evaluate only the effect of high-melting-point inclusions such as alumina, spinel, and complexes thereof, and thus cold workability and fatigue properties are not necessarily evaluated correctly and accurately.
Meanwhile, the technique disclosed in Japanese Patent Application Laid-Open (kokai) No. 151749/1995 pays no attention to conditions for energy irradiation during EB melting. Therefore, the method for evaluating cleanliness disclosed in the publication involves the same problem as in the above-described technique disclosed in Japanese Patent Application Laid-Open (kokai) No. 40082/1993. That is, the obtained evaluation may reflect not only the effect of high-melting-point inclusions, such as alumina, spinel, and complexes thereof, but also the effect of low-melting-point inclusions such as MnS and SiO2, and thus cold workability and fatigue properties is not necessarily evaluated correctly and accurately.
In view of the foregoing, an object of the present invention is to provide super-clean steel exhibiting excellent cold workability and fatigue properties, by quantitatively confirming the effect of high-melting-point inclusions such as alumina, spinel, and complexes thereof, which are considerably detrimental to cold workability and fatigue properties.
The gist of the present invention resides in the following:
super-clean steel in which the area of non-metallic inclusions existing in the surface of a sample of the steel is not more than 15,000 xcexcm2 per gram, when the sample is melted by an electron-beam under the following conditions (per gram): an energy irradiation rate of 200-600 J/second, an irradiation time of 10-25 seconds, and an irradiation energy of 5,000 J or more, and then solidified.
The area of non-metallic inclusions present in the surface of the sample after solidification can be measured, for example, by observing the back-scattered electron image of the inclusions with a scanning electron microscope, and analyzing the electron image transmitted into an image processing apparatus.
Electron-beam melting (EB melting) is performed by irradiating a sample with an EB under vacuum atmosphere.